Japanese Patent Application Publication No. H7-146263 discloses a combustion type water heater that heats water by burning fuel. This combustion type water heater is provided with a burner that generates combustion gas by burning fuel, a heat exchanger that exchanges heat between the water passing through inside thereof and combustion gas flowing outside thereof, and a fouling sensor that detects whether or not scale is built up inside the heat exchanger. According to this combustion type water heater, the fouling sensor detects scale buildup inside the heat exchanger and treatment for descaling can be performed.